In the commercial concentrated solar power (CSP) applications, sensible thermal energy storage (STES) for the high temperature solar collectors is available for temperatures up to 565° C. For example, the solar salt a binary eutectic mixture of KNO3 (54 wt. %) and NaNO3 (46 wt. %) has been mainly used as a thermal energy storage (TES) medium for the parabolic trough collectors with a synthetic oil employed as a heat transfer fluid (HTF) at temperatures up to 400° C. For the power tower, the solar salt is used as a TES medium as well as a HTF up to 565° C. For thermal energy above 565° C., the solar salt is not suitable for the service, because it becomes thermally unstable above this temperature.
The solar salt in molten liquid state has been used only as a TES medium for the parabolic trough collectors, but not as a HTF, because the solar fields of the parabolic trough collectors have complex HTF distribution piping that makes it difficult to provide with a means for freezing protection for the molten solar salt that has a melting temperature of 222° C. Therefore, synthetic oil such as Dowtherm A having a melting point of 12° C. has been widely used as a HTF. This synthetic oil can be used only up to 400° C., however, because it becomes thermally unstable above this temperature.
The molten solar salt has been used as both a TES medium and also a HTF for the power tower receiver that produces thermal energy above 500° C. for liquid HTF. As the HTF piping between the power tower receiver and the thermal energy storage tank is relatively simple in this case, it has been possible to provide the HTF piping with a freezing protection capability. The solar salt, however, becomes thermally unstable from around 565° C., and cannot be used above this temperature. Thus, a new heat storage process is necessary for the storage temperatures above 565° C. using a TES medium other than the solar salt. As for the HTF, a new heat transfer process is also necessary for the service temperatures above 400° C. using a thermally stable HTF other than the synthetic oil.
The candidates for the new TES medium include the eutectic mixtures of carbonate, chloride, and fluoride alkali metal compounds that can be used between 565° C. and 850° C. Unlike the solar salt, they are very corrosive making it more costly to employ the process equipment units constructed of high nickel alloys that are compatible with the molten salt liquids. Also, the carbonate salts are thermally unstable at the service temperatures.
The candidates for the new HTF can be found from the experiences in the nuclear power industry. As a liquid HTF, molten metal or metal salts such as Na, Pb and NaK have been used as a HTF, but have low specific heat making it difficult to be used as a TES medium at the same time. Na and NaK are also very flammable when spilled, so the safety is a major concern. Molten eutectic salts such as LiF—NaF—KF (46.5-11.5-42 mole %) and NaF—NaBF4 (8-92 mole %) have been tried as a liquid HTF for the molten salt nuclear reactors at temperatures around 700° C. In this case, the process equipment surfaces contacting the HTF must have been constructed of high nickel alloys for corrosion protection. This need makes the fluoride salts too expensive to be used for CSP plants. As for a gaseous HTF, simple inorganic gases such as He and CO2 have been used at high pressures. The gases have very low heat capacities, and cannot provide a cushion even for the disturbances from the heat sources such as those caused by clouds in solar irradiation. At the present time, by using a tubular receiver, the power tower receiver can heat a gaseous HTF above 800° C. and a liquid HTF above 600° C. at high pressures as indicated by Ho and Iverson (2014).
Carbonate alkali metal salts have been studied by Petri et al. (1980) in search of TES materials in high temperature applications. It was found that the decomposition is minimized on molten Li2CO3, Na2CO3 and K2CO3 by maintaining a finite partial pressure of CO2 gas. Chen et al. (2014) also reported on the thermal stability of eutectic ternary mixture of Li2CO3—Na2CO3—K2CO3 in a composition of 32, 33 and 35 wt. %, respectively, having melting temperature of 397° C. that it was thermally stable up to 1000° C. in the CO2 gas atmosphere.
In addition, Petri et al. (1980) tested the molten salt of a ternary eutectic mixture Li2CO3—Na2CO3—K2CO3 with a Duocel aluminum foam in aluminum crucibles at 450° C. The aluminum foam was found very compatible in corrosion with the molten salt liquid forming a thin protective layer, possibly of LiAlO2, on the metal surfaces.
Lately, Glatzmaier and Gomez (2015) have suggested aluminum coating for use in storage tanks and process equipment units in the CSP applications at temperatures up to 850° C. Their analysis shows that the aluminum coated equipment would cost about twice as much as the stainless steel, whereas the high nickel alloys compatible with the molten carbonate salts sometimes would do more by an order of magnitude than the stainless steel.
As for the thermal stability of the CO2 gas, it is stable at the service temperatures up to 1000° C. For example, the decomposition composition of a pure CO2 gas at 1000° C. is 0.011% at 1 bar, 0.005% at 10 bar, and 0.0024% at 100 bar.
The material of construction for the CO2 gas at high temperatures is also reported. For the service temperatures below 450° C., low alloy, temperature resisting, ferric steels containing molybdenum, chromium, or both can be used; in temperatures between 300° C. and 700° C., austenitic stainless steels; and in temperature up to 1000° C., Ni—Cr or Ni—Cr—Fe alloys.
According to Bauer et al. (2016), the SunShot Initiative is supporting a research for development of a power tower receiver for directly heating supercritical CO2 at pressures up to 250 bar and temperatures up to 700° C. Even though the supercritical CO2 at these conditions can be used as a working fluid for the supercritical CO2 (sCO2) Brayton cycle, this material cannot be utilized as a TES medium.
SunShot Initiative also supports the development of a eutectic binary mixture of MgCl2-KCl with a Mg metal additive as a corrosion inhibitor, according to Bauer et al. (2016). Once the research becomes successful in commercial applications, the MgCl2—KCl eutectic mixture having melting temperature of 435° C. can be readily utilized in the multi-chamber STES system of this invention where the Mg additive slurry can be bubbled with an inert gas in the chambers.
As for the applications requiring an immiscible HTF above 400° C. where the organic synthetic oil can no longer be used, the simple inorganic gases such as air, Ar, He, N2 and CO2 are the only choices at the moment. The gaseous HTF's, however, usually require very high volumetric flow rates at moderate pressures due to their low volumetric specific heat capacity limiting their use.
The bubble columns, where the mass transfer and heat transfer for chemical reactions are promoted by introducing the gaseous reactants into a slurry layer, have been used for volumetric heat transfer coefficients up to around 60 KW/m3·° C. by using the submerged heat exchangers as described by Zehner and Kraume (2005). Even in this case, a large portion of sensible heat must have been transferred by direct contact heat transfer between the gas bubbles and liquid layer before the fluid mixture contacts the heat exchanger tube surfaces. When the required volumetric heat transfer coefficient is very low such as for the STES system for CSP plants with the heat transfer coefficients of around 3 KW/m3·° C. needed, only the direct contact heat transfer can be used. This heat transfer method obviates the need of a heat exchanger between the hot gaseous HTF stream from the power tower receiver and the heat storage medium of corrosive molten salt.
Numerous such bubble columns are presently operating worldwide for chemical synthesis in petrochemical industries. The Fisher-Tropsch reactors, for example, have been operating in sizes of up to 10 meters in diameter and 40 meters in height at operating temperatures of around 250° C. and pressures of 20 to 40 bar for more than a half century. The reactor takes as feedstocks a gaseous mixture of carbon monoxide and hydrogen into a hydrocarbon liquid layer containing metal catalyst particles. Also, the ebullated bed reactors for hydroconversion of petroleum residua operate in sizes of up to 5 meters in diameter and 30 meters in height at around 450° C. and 240 bar. The reactors use hydrogen gas and petroleum residua liquid as feedstocks with metal catalyst particles. As a matter of fact, they are the most reliable reactors being in use for the chemical industry.
At the present time, there is no reliable TES system available for the commercial CSP plants at service temperatures above 565° C. the highest temperature at which the molten solar salt can be used. In order to increase the cycle efficiency of the thermodynamic cycle engines, however, the engines need to operate at temperatures higher than this limit. For example, the sCO2 Brayton cycle engine can achieve the cycle efficiency of 50% at around 700° C., whereas the steam Rankine cycle presently operating with the solar salt at 565° C. achieves cycle efficiency of around 40%. Since the solar power towers can generate thermal energy with the gaseous HTF at temperatures higher than 800° C. and with the liquid HTF higher than 600° C., new methods must be devised to harness the thermal energy of such quality.